This invention relates generally to a method and apparatus for media blasting and more specifically relates to an improved method and apparatus for media blasting a workpiece or part. This invention uses the powered part hold-down apparatus of U.S. Pat. No. 5,272,897, the disclosure of which is hereby incorporated by reference.
In the finishing of machined or fabricated parts or workpieces, the use of media blasting or peening to increase the fatigue strength of a workpiece or part is well known. An example of such a part is a gear utilized in automobile transmissions. Once these gears are formed, they should be processed by media blasting machines and the like to ensure that the gears meet government and manufacturer specifications, as well as to ensure that the gears are suitable for performing their intended functions.
Usually, the workpiece or part is placed in a closed chamber, in which a blasting means or a blasting system is located. A typical blasting system is a suction type system, which includes a media feed hose and an air supply hose interconnected at a suction gun. When the blasting system is actuated, the suction gun is turned on. This, in turn, causes media to be drawn from a storage hopper and into the media feed hose. The media are drawn through the media feed hose to the suction gun, where the media are then mixed with air from the air supply hose. After mixing of the media and air, the suction gun directs a stream of the air/media mixture against the workpiece, to provide compression stress to the surface by means of the colliding media.
Media blasting operations/systems of this type have a number of problems. Although media blasting strengthens the workpiece, it may also roughen the surface of the workpiece and/or result in an uneven surface finish. Surface roughness or an uneven surface finish decreases the hardness of the workpiece and can interfere with its intended function. For example, surface roughness or uneven surface finish in a gear interferes with the ability of the gear to mesh. Moreover, a workpiece having an uneven surface finish is susceptible to crack propagation, which can result in workpiece failure. Thus, it is important that the surface finish be evenly dimpled with a uniform distribution of peaks and valleys, as an uneven distribution contains deeper valleys which can propagate as cracks.
A further problem with media blasting operations/systems of this type result from the use of suction means to provide the media blast stream. As discussed above, in media blasting or shot peening operations, the dimple formation on the workpiece must be carefully controlled. In regulating dimple formation, the velocity of the media is an important control parameter. However, with suction guns it is difficult to control the flowrate of the media blast stream. It is also difficult to adjust the flowrate when changing from use of one type of media to another type, as the air velocity and media flow rate cannot be adjusted independently. Moreover, in a suction gun the media and air are mixed immediately prior to ejection, which may not allow the media to develop sufficient velocity to provide desired compressive stress.
Prior art methods have attempted to solve the problems of surface roughness or uneven surface finish by using glass beads. Although glass beads tend to reduce surface roughness, they may not sufficiently increase the fatigue strength of the workpiece. A further problem with this and other prior art methods is the difficulty of obtaining even peening of the workpiece surface. This problem arises, in part, from the lack of directivity of the media. Prior art methods also suffer from a high wastage of media, due to attrition of the media during the blasting process. The high wastage of media reduces both the operational efficiency and cost-effectiveness of a blasting operation.
Japanese patent application number H6-217214, entitled "A Strengthening Method For Metal Parts," filed Sep. 12, 1994, in the name of Honda Motor Company, Ltd., describes a media blasting method which attempts to alleviate some of the above mentioned problems. This Japanese application describes a metal strengthening method in which a stream of water and glass beads is directed toward a metal part. Use of water as a suspending medium imparts directivity to the glass beads. It also provides additional compression stress and provides a polishing effect.
Although use of water improves the surface smoothness of the workpiece and the directivity of the media, it introduces additional problems without addressing the problem of media wastage or media flow control. In particular, use of wet media complicates design of the media blasting system, because all the parts of the media blasting system have to be designed for the presence of water. For example, care must be taken to avoid creating an electrolytic junction. Use of water also limits the type of workpieces that can be processed and the type of media which can be blasted. For example, it would be undesirable to use this method with a workpiece which is susceptible to corrosion.
Accordingly, a media blasting apparatus and method according to the teachings of the present invention is intended to solve problems inherent in the prior art media blasting systems discussed above, as well as others. A media blasting system and method according to the present invention increases operational efficiency, reduces media wastage and media use, provides even peening of a workpiece surface, provides a polishing effect or an even surface finish, and, moreover, obviates use of a high directivity media stream.